Silicone coating compositions can be applied to any substrates which are to be rendered repellent to tacky substances, and can be subsequently crosslinked to give an elastomeric silicone coating. The high level of repellency of tacky substances is one of the specific characteristics of silicones, particularly materials containing a considerable proportion of diorganosiloxane units. An essential constituent of all silicone coating compositions, consequently, is an organopolysiloxane which may, depending on the nature of crosslinking, have been functionalized. Catalysts, crosslinkers, photosensitizers, solvents, emulsifiers, fillers, resins, etc. may also be present. The crosslinking of the applied silicone coating composition to give an elastomeric silicone coating can be carried out and/or accelerated thermally, by means of high-energy particles or electromagnetic radiation, or by a combination of these methods. Thermally induced addition crosslinking through reaction of alkenyl-functional organopolysiloxanes with SiH-containing crosslinkers, in particular by platinum-catalyzed hydrosilylation, has acquired considerable practical significance. In addition to those coatings which crosslink by addition reaction, silicone coating compositions have also been described which crosslink by condensation, in the presence of peroxides, and by radiation.
Silicone coating compositions may also differ in their content of solvents, and in other manners as well. For example, the crosslinkable silicone coating compositions can include solvent or preferably be free from solvent, or may be present in the form of an aqueous emulsion.
The degree of repellency of tacky substances by the silicone coating can be quantified by the release force. The release force is a measure of the force which must be exerted in order to maintain delamination of a standardized adhesive bond at a defined rate of release, in accordance, for example, with the FINAT test methods. The test material may comprise, for example, a laminate of a siliconized release paper and an adhesive-coated test strip. The greater the release force measured, the more firmly the self-adhesive material adheres to the siliconized release paper. Although numerous factors such as temperature, rate of release, peel angle, film thickness of the adhesive material, pressure applied when producing the adhesive bond, etc. are also important, the release force depends predominantly on the nature of the tacky substance and on the type of the silicone coating.
Since the nature of the tacky substance is predetermined by the particular application, it is particularly important to be able to tailor the release properties of the silicone coating, i.e. to adapt them to the performance requirements. For example, siliconized release papers that are to be used for packaging tacky materials or for placing under dough products during the baking procedure should have a very high level of repellency for tacky substances, corresponding to very low release force values. In contrast, siliconized release papers used as a backing for labels must--depending on the size of the label paper--possess a relatively high level of release force, since otherwise the label might become prematurely detached from the backing.
Obtaining suitable release requirements for silicone coatings is difficult. In general, there is not only the need for a certain level of release force associated with a specific adhesive but moreover, this release force should also show a predetermined dependency on the rate of release. For example, the release force of the relatively small labels used to price consumer goods should be as high as possible in the state of rest and at a low peel rate, so as to preclude premature detachment in the dispensing device, whereas at a high rate of peel the release force should be as low as possible in order to enable mechanical pricing to take place rapidly and without tearing. Other applications require the converse behavior, namely high release force at high rate of peel, or release force behavior independent of the rate of release, also referred to as the peel rate. Specific requirements relating to the temperature dependency of the release force level are set, for example, for label papers suitable for laser printers.
Reproducibly tailoring the dynamic release force behavior of a silicone coating irrespective of its storage period, therefore, represents a decisive criterion for its applicability.
As a general rule, silicone coatings have a very low level of release force; in other words, the extent of the repellency of tacky substances is very high. Consequently, the problem of tailoring the release behavior is principally that of effecting an increase in release force. This targeted increase in release force is referred to as "controlled release" (CR), and the additives used for this purpose are known, correspondingly, as "controlled-release additives" (CRAs).
Numerous possibilities belonging to the prior art for tailoring, and in particular increasing, the release force level can be given. It is known, for example, that an increase in the proportion by weight of organic groups in the organopolysiloxane, which may be achieved by replacing some of the methyl groups by phenyl groups or other organic radicals of relatively high molecular mass, is associated with an increase in the release force values with respect to a range of tacky substances. The result is that an increase in the organic nature of the silicone rubber composition is accompanied by suppression of the silicone-typical release properties, thereby reducing the extent of repellency of tacky substances.
The organic nature of a silicone coating composition can be increased by various means. For example, it can be done by increasing replacement of the methyl groups, which are typically present in the organopolysiloxane, by organic radicals of relatively high molecular mass and, in particular, relatively high polarity. An alternative option is to incorporate divalent organic radicals into the siloxane main chain. Furthermore, the organopolysiloxane can be replaced in part by organic compounds, especially organic oligomers or polymers, which may contain functional groups to enable them to be incorporated by crosslinking into the silicone network, and at the same time to prevent unwanted exudation at the surface of the silicone coating.
Although such methods of regulating the release force are possible in principle, employing them is accompanied by numerous disadvantages. For instance, it is necessary to modify the organopolysiloxane forming the principal constituent, in which case the organic content in the organopolysiloxane must be increased drastically in order to obtain high release force values. The preparation of organopolysiloxanes having organic radicals of relatively high molecular mass, or of block copolymers, etc., however, is highly complex. Owing to different types of adhesives to be employed, and to different release force requirements, moreover, it would be necessary to provide a range of variously modified organopolysiloxanes. The prospects for release force regulation through the addition of organic compounds, especially those of relatively high polarity, to the silicone coating composition are equally poor, since massive compatibility problems occur which prevent the reproducible adjustment of the level of release force independently of the storage period. Such additions may, moreover, impair the crosslinking reaction necessary to cure the coating.
Particular significance as CRAs has been acquired by resinous additives essentially comprising SiO.sub.4/2 and/or RSiO.sub.3/2 units, and also R.sub.3 SiO.sub.1/2 and possibly R.sub.2 SiO.sub.2/2 units, where R is identical or different and can be --H or --OH or any substituted or unsubstituted organic radical. EP-A-652 258 describes silicone resins, and U.S. Pat. No. 4,611,042 discloses resinous siloxane copolymers, as CRA's. Disadvantages which the CRA's are manifest. For example, release force is increased markedly only at a very high content of silicone resin; silicone resins are expensive; and silicone resins cause an considerable and unwanted increase in the viscosity of the silicone coating composition, which must be compensated by choosing shorter-chain silicone polymers and/or by the use of undesirable solvents. Further, silicone resins frequently increase the extractables content of the crosslinked silicone film, which is manifested in lower values for residual adhesion. For example, a price label which is peeled off from such a siliconized paper adheres poorly to the product article because the adhesive of the label becomes contaminated with silicone. Finally, silicone resins are frequently unable to provide a low release force at low rate of peel and a high release force at high rate of peel.
The preparation of organopolysiloxanes comprising T and/or Q units is relatively complex. There is, in particular, a risk of gelling. The use of silicone resins also impairs the vulcanization characteristics, i.e. these resins reduce the rate of crosslinking. A very short crosslinking time, however, is critical for the majority of applications of silicone coating compositions, such as the siliconization of release papers. The advantage of the ready availability of linear organopolysiloxanes is abandoned when silicone resins are employed.
U.S. Pat. No. 5,545,831 describes a linear organo-siloxane which does not necessarily contain SiH but necessarily includes alkylene groups between Si atoms, which can be employed as a CRA for silicone coating compositions without impairing the residual adhesion. The SiH in the CRAs disclosed is present, if at all, only at the chain ends. The chain-end SiH group, which may also be an alkenyl group, apparently serves to incorporate the CRA's by crosslinking; the increase in release force is effected by the alkylene groups, i.e. by the incorporation of organic radicals. A disadvantage is that CRAs of this kind bring about a marked increase in release force only when the CRA content is high.
EP-A-605 227 discloses a process for regulating release force which is based on tailoring the crosslinking density and/or the modulus of the silicone coating. In particular, an increase in release force at high peel rate is achieved by combining an alkenyl-terminal organopolysiloxane with an SiH-terminal organopolysiloxane, which owing to the chain extension that takes place in the course of crosslinking leads to a silicone film having a relatively low nodal density and relatively low modulus and hence a relatively high release value at high peel rate. Disadvantages of this method are that regulation of release force can be achieved only at high peel rate, and is variable, moreover, only within narrow margins, since changes in the nodal density also affect the mechanical strength, especially the abrasion resistance. In addition, as the proportion of SiH-terminal organopolysiloxanes increases, the residual adhesion is adversely affected. It is expressly stressed in the disclosure of EP-A-605,227 that the additionally required SiH crosslinker must be soluble in the silicone coating composition.
EP-A-355 381 describes controlling the release force behavior of silicone coatings by using an optionally SiH-containing organosiloxane which possesses a sufficient number of monofunctional phenolic structural units.
DE-A-25 09 620 describes a method of regulating the release force for addition-crosslinking silicone coating compositions whose principal constituent comprises diorganopolysiloxanes (I) in which from 3 to 30 mol-% of the nonterminal siloxane units are diphenylsiloxane units and at least 50% of the number of organic radicals in the remaining siloxane units are methyl radicals. By combining these phenyl-containing diorganopolysiloxanes (I) with organopolysiloxanes (II) which carry at least three Si-bonded hydrogen atoms it is possible, owing to the different concentration of Si-bonded hydrogen in the organopolysiloxane (II), to alter the release force of the resulting silicone coating continuously within wide limits. What proves disadvantageous here, inter alia, is the need to use phenyl-containing organopolysiloxanes as a principal constituent and to provide a relatively large number of different SiH-containing organopolysiloxanes (II).